I have made an invention in the technical area of soil dynamics, which is a method for making pre-construction estimates of the frequencies of foundations under dynamic machinery and equipment. It is known that the natural frequencies of foundations are the dominant parameters in design and specification of foundations for machinery having dynamic loads. Knowledge of the exact values of these frequencies allows more accurate computation of vibration amplitudes and enables engineers to avoid the conditions of resonance of foundation vibrations.
The calculated natural frequency for foundations can be obtained on the basis of dynamic features of the foundation-soil system: mass, stiffness, and damping. There is no singular, generally accepted opinion on the nature of these features. However, there have been many theoretical and experimental approaches for determination of the characteristics of the foundation-soil system, but the results of application of the various methods frequently do not give coincident results. See, for example, Barkan, D. D. (1962), Dynamics of Bases and Foundations, McGraw-Hill Book Co., New York; Richart, F. E., Hall, J. R., and Woods R. D. (1970), Vibrations of Soil and Foundations, Prentice-Hall, Inc., Englewood Cliffs, N.J.
Furthermore, dynamic loads on a soil base effectively induce elastic waves in a soil base. Methods for in-situ surface measurements of steady-state and transient soil vibrations are used for examinations of elastic soil properties. The spectrums of soil vibrations excited by impacts show a few maximums with the dominant frequency of surface waves.
Frequencies of elastic waves are stable quantities which have been found to be characteristic for the site investigated. Actually, these frequencies are the natural frequencies of the soil bases and the values obtained do not practically depend on conditions at the contact area where impacts are made directly on the soil.
See, also, the following patents.
U.S. Pat. No. 3,224,253, to McKay, discloses an apparatus for conducting tests for analysis of existing structures in situ.
U.S. Pat. No. 3,391,571, to Johanson, is an apparatus and method for determining the effectiveness of vibratory devices.
U.S. Pat. No. 3,946,598, to Towne, et al., pertains to measuring the dynamic parameters of soil by driving piles into that soil.
U.S. Pat. No. 4,856,318, to Hogan, et al., reveals a method and apparatus for determining the suitability of playground equipment.
SU Patent 0822024 discloses an apparatus for driving steel casings into soil for dynamic sounding. The standard SPT Test measures the number of impacts and the depth to which the casing is driven downward by a series of impacts.
SU 1629403 applies to impacts on frozen ground, before and during the thawing process and further measurements upon complete thawing of the ground.
U.S. Pat. No. 4,128,011 to Savage, disclosed a method and apparatus for the investigation of the soundness of existing structures.
Accordingly, a need exists for a method to estimate, before construction, the natural frequency of damped vibrations of a designed foundation under operative machinery loads. And therefore, the primary objective of this invention is the disclosure and teaching of such a method.
The method disclosed herein is based on the relationship between the natural frequencies of vertical damped vibrations of rigid bodies on soil bases and the natural frequencies of the soil bases. Use of the method provides a means for predicting by essentially accurate estimates the natural frequencies for machine foundations prior to their erection at a specific site.